Method for the catalytic oxidation of volatile organic compounds

ABSTRACT

A catalyst for the full oxidation of volatile organic compounds (VOC), particularly hydrocarbons, and of CO to CO 2 , comprising:
         a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to A 14 Cu 24 O 41  (I), where A is Sr or a solid solution of Sr with alkaline-earth metals, alkaline metals, lanthanides; or a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to B 4 Cu 5 O 10  (II), where B is Ca or a solid solution of Ca with alkaline-earth metals, alkaline metals, lanthanides; or mixtures thereof; and in that it is prepared in a form which has a large specific surface area, preferably larger than 25 m 2 /g;   a method for preparing the catalysts; their use in methods for the full oxidation of VOC and of CO to CO 2 ; and the oxidation methods.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 09/590,454 whichis now U.S. Pat. No. 6,638,492 filed on Jun. 9, 2000.

FIELD OF THE INVENTION

The present invention relates to catalysts for the full oxidation ofvolatile organic compounds (VOC), particularly hydrocarbons, and to amethod for the full oxidation of volatile organic compounds (VOC) byusing said catalysts.

BACKGROUND OF THE INVENTION

The total combustion of VOC to CO₂ and H₂O becomes necessary in view ofthe toxicity and environmental impact of most unburnt VOC. The goal isto minimize the release of VOC into the atmosphere and the forming ofCO, which is in turn a toxic component.

The catalysts used most for VOC combustion are:

a) catalysts based on noble metals, which are characterized by a highcost but also by excellent performance in terms of VOC conversion, andoperate at temperatures from 200 to 450° C. according to the reactivityof the compound;

b) catalysts based on mixed oxides, typically chromites of copper or ofother metals, or barium hexaaluminate, which are characterized by alower cost but are active in more drastic conditions (temperatures from400 to 600° C.). This second class of catalysts is also used forcatalytic combustors for power generation units. In this case, theyoperate at temperatures above 900° C.

The types of catalyst used for the combustion of VOC are not free fromdrawbacks; high cost (for those based on noble metals) and poor activity(for the second class, accordingly requiring operation at highertemperatures, in conditions in which morphologic or structuraltransformations are facilitated).

SUMMARY OF THE INVENTION

The aim of the present invention is to eliminate the drawbacks of knowntypes of catalyst for the full oxidation of VOC.

In particular, an object of the present invention is to providecatalysts for catalytic combustion which are characterized by highactivity, high resistance to temperature, extreme operating conditions,low cost and easy production even as composites and thin films.

Another object of the present invention is to provide catalysts for VOCoxidation with high selectivity for carbon dioxide CO₂ with respect tocarbon monoxide CO.

Another object of the present invention is to provide catalysts whichlead to full oxidation of the VOC in stoichiometric andnon-stoichiometric mixtures of VOC and oxygen (oxidizing or reducingconditions), so that the mixture of gases produced due to VOC oxidationcontains no carbon monoxide but contains only carbon dioxide.

Another object of the present invention is to provide a method for thefull oxidation of VOC which avoids carbon monoxide removal operationsand the known negative consequences of the presence of carbon monoxidein the environment.

Another object is to provide a method for oxidizing VOC to CO₂ whichutilizes the full potential of the VOC oxidation reaction, with evidentenergy-related advantages.

Another object is to provide a method for eliminating carbon monoxidefrom gas mixtures that contain it together with oxygen.

BRIEF DESCRIPTION OF THE INVENTION

This aim, these objects and others which will become apparent from thedetailed description of the invention are achieved by catalystsaccording to the present invention, which comprise one or morenon-stoichiometric crystalline compounds conventionally referenced byformulas which respectively correspond to:A₁₄Cu₂₄O₄₁  1)B₄Cu₅O₁₀ (BCuO₂ is also cited in the literature)  2)and by a method for oxidizing VOC and CO to CO₂ according to the presentinvention, which uses the same catalysts.

In the first of the above cited formulas, A is Sr or a solid solution ofSr with alkaline-earth metals, alkaline metals, lanthanides; in thesecond formula, B is Ca or a solid solution of Ca with alkaline-earthmetals, alkaline metals, or lanthanides.

DETAILED DESCRIPTION OF THE INVENTION

Examples of catalysts according to the invention have the approximateformulaSr₁₄CU₂₄O₄₁orCa₄Cu₅O₁₀

Both of these compounds and their derivatives by substitution are widelyknown in the literature (for Ca₄Cu₅O₁₀ (mentioned as CaCuO₂): Roth etal, J Am Ceram Soc, Vol. 72, p. 1545 (1989); for Sr₁₄Cu₂₄O₄₁: McCarronet al, Mat Res Bull, Vol. 23, p. 1355 (1988)), and particularly for thecompound Sr₁₄Cu₂₄O₄₁ there is a vast body of literature associated withits unusual electronic properties. Although it is not possible toformulate exactly the above components, they are unequivocallydistinguished by their chemical-physical properties and particularly bythe powder diffraction profiles, which correspond to the ones listed inJCPDS international powder diffraction tables, on cards 43-0025 and46-0054 for the compounds referred to as Sr₁₄Cu₂₄O₄₁ and CaCuO₂,respectively.

The fixing properties of said compounds and derivatives thereof havebeen described in patent application BO98A000593 herein incorporated byreference. The same patent application describes the use of saidcompounds to fix gases and gas fixing devices which comprise saidcompounds.

The inventors have now found that surprisingly said compounds, ifprepared so as to have a large specific surface area, preferably largerthan 25 m²/g (as measured by the BET method), act as catalysts for VOCoxidation. The inventors have found that the reaction for full oxidationof VOC in the presence of the catalysts according to the inventionoccurs with a high conversion of VOC even at low temperatures.

Moreover, the inventors have found that the catalysts according to thepresent invention allow VOC oxidation (even when the conversion ispartial) with total selectivity toward the forming of CO₂ even inconditions in which there is a significant deficit of oxygen withrespect to the stoichiometric mix. The expression “total selectivity” isused to reference the fact that VOC oxidation occurs until only CO₂ andH₂O are obtained. In other words, in conditions of oxygen deficit, whilethe quantity that corresponds stoichiometrically to the quantity ofoxygen that is present is converted into CO₂ and H₂O, the other fractionof VOC remains unconverted.

The temperature ranges over which the catalytic oxidation process isactive depend crucially on the volatile organic compound to be oxidized.Considering methane as the most stable and least easily oxidizablehydrocarbon, the activation temperatures of the reaction for fulloxidation of methane constitute the upper limit of the activationtemperatures for any VOC: in particular, the activation temperatures ofmethane are from 300° C. to 350° C. and from 350° C. to 400° C. for thecompounds A₁₄Cu₂₄O₄₁ and B₄Cu₅O₁₀, respectively. The maximum utilizationtemperatures of the catalysts instead correspond to the decompositiontemperatures of the compounds A₁₄Cu₂₄O₄₁ and B₄Cu₅O₁₀, which areproximate to 1000° C. and 750° C., respectively.

The methods for full oxidation of VOC according to the present inventionmay be performed in combustion chambers or in reheat chambers or fluegas chambers.

The catalytic oxidation reaction of the catalysts according to thepresent invention occurs on a fixed bed or on a fluid bed.

The catalysts according to the present invention can be in the form ofgranules.

Advantageously, the catalysts according to the present invention includea substrate material. The substrate can be an inert substrate in theform of a thin film or a composite material. Preferably, the substratematerial is constituted by porous substrates which are inert withrespect to the above described active materials, such as Al₂O₃, TiO₂,ZrO₂, CeO₂, MgO, on which the active material is deposited byimpregnation with the aqueous solution of soluble salts (for examplenitrates or citrates or acetates or mixtures thereof) of the constituentmetals in the correct stoichiometric ratios.

The catalysts according to the invention preferably comprise 5 to 20% byweight of a non-stoichiometric crystalline compound, conventionallydesignated by a formula which corresponds to A₁₄Cu₂₄O₄₁ (I), where A isSr or a solid solution of Sr with alkaline-earth metals, alkalinemetals, lanthanides; or a non-stoichiometric crystalline compoundconventionally designated by a formula which corresponds to A₄Cu₅O₁₀(II), where A is Ca or a solid solution of Ca with alkaline-earthmetals, alkaline metals, lanthanides; or mixtures thereof.

A catalyst comprising a non-stoichiometric crystalline compoundconventionally designated by a formula which corresponds to Sr₁₄Cu₂₄O₄₁can be prepared, for example, with a method according to the inventionwhich comprises the steps of:

a) immersing a pre-dried granular porous substrate material in anaqueous solution with a molar concentration of Sr(NO₃)₂ from 0.23 M to0.93 M and a molar concentration of Cu(NO₃)₂ from 0.39 M to 1.59 M;

b) drying at a temperature from 80° C. to 120° C.;

c) holding at a temperature from 650° C. to 750° C. in a gas streamwhich contains oxygen until complete decomposition of the nitratesoccurs.

A catalyst comprising a non-stoichiometric crystalline compoundconventionally designated by a formula which corresponds to Ca₄Cu₅O₁₀ isprepared with a method according to the invention which comprises thesteps of:

a) immersing a pre-dried granular porous substrate material in anaqueous solution of Ca(NO₃)₂ and Cu(NO₃)₂ in an equimolar ratio and at amolar concentration from 0.39 M to 1.39 M;

b) drying at a temperature from 80° C. to 120° C.;

c) holding at a temperature from 650° C. to 750° C. in a gas streamwhich contains oxygen until complete decomposition of the nitratesoccurs.

Furthermore, a catalyst comprising a non-stoichiometric crystallinecompound conventionally designated by a formula which corresponds toCa₄Cu₅O₁₀ is prepared with a method according to the invention whichcomprises the steps of:

a) immersing a pre-dried granular porous substrate material in anaqueous solution obtained by dissolving, with the application of heat,CuO and CaCO₃ in nitric acid, so that the molar ratio between thecomponents of the solution is CuO:CaCO₃:HNO₃=1:0.83:3.2; water andcitric acid is added thereto so that the citric acid: Cu molar ratio isfrom 3.5:1 to 4.0:1;

b) heating in air until combustion of the organic fraction of theabsorbed material is achieved;

c) thermal treatment for 4 to 24 hours at a temperature from 650 to 750°C. in a stream of gas containing oxygen.

Preferably, the porous material that is used is constituted by Al₂O₃,ZrO₂, CeO₂, TiO₂, MgO.

However, one should not consider the present invention to be limited tocatalysts prepared with the described methods.

EXAMPLES

The catalysts according to the present invention and the oxidationmethod according to the present invention are described in greaterdetail hereinafter with examples which are given only by way ofnon-limitative illustration.

Examples of Preparation

1) After drying at 150° C. for 12 hours, an appropriate amount of porousalumina with a particle size from 2 to 4 mm, specific surface area ofapproximately 400 m²/cm³ and capable of absorbing at least 1 ml ofsolution per gram is immersed in an aqueous solution of Sr(NO₃)₂ and ofCu(NO₃)₂, at a concentration of 0.556 M and 0.952 M, respectively. Theamount of alumina used should be such as to absorb almost all thesolution. The alumina impregnated by the solution is then dripped, driedat 80° C. for 4 hours, and finally treated at 650° C. in a stream of airor oxygen for 24 h. This method produces a catalyst which contains 12%by weight of Sr₁₄Cu₂₄O₄₁ compound and has a specific surface area inexcess of 200 m²/cm³.

2) After drying at 150° C. for 12 hours, an appropriate amount of porousalumina with a particle size from 2 to 4 mm, specific surface area ofapproximately 400 m²/cm³ and capable of absorbing at least 1 ml ofsolution per gram is immersed in an aqueous solution of Ca(NO₃)₂ and ofCu(NO₃)₂, in an equimolar ratio and at a concentration of 0.556 M. Theamount of alumina used should be such as to absorb almost all thesolution. The alumina impregnated by the solution is then dripped, driedat 80° C. for 4 hours, and finally treated at 650° C. in a stream of airor oxygen for 24 h. This method produces a catalyst which contains 7% byweight of Ca₄Cu₅O₁₀ compound and has a specific surface area in excessof 200 m²/cm³.

3) After drying at 150° C. for 12 hours, an appropriate amount of porousalumina with a particle size from 40 to 80 mesh, specific surface areaof approximately 400 m²/cm³ and capable of absorbing at least 1 ml ofsolution per gram is immersed in an aqueous solution obtained bydissolving, with the application of heat (80° C.), one mole of CuO and0.83 moles of CaCO₃ in nitric acid so that the nitric acid/copper oxidemolar ratio is 3.2, water and citric acid being added thereto, in orderto complete dissolution of the reagents, so that the citric acid/Cumolar ratio is 3.8. The amount of alumina used should be such as toabsorb almost all the solution. The alumina impregnated by the solutionis then dripped and heated in air until combustion of the organicfraction of the absorbed material is achieved. The resulting material isthen subjected to thermal treatment at 700° C. in a stream of air oroxygen for 24 h. This method produces a catalyst which contains 11% byweight of Ca₄Cu₅O₁₀ compound and has a specific surface area in excessof 200 m²/cm³.

Example 1

Sr₁₄Cu₂₄O₄₁: Catalytic Combustion of Methane

A catalyst containing Sr₁₄Cu₂₄O₄₁ was used in catalytic methanecombustion tests.

The catalyst was a composite material constituted by an inert poroussubstrate (Al₂O₃) containing 12% by weight of active compound (60 mg).

The tests were conducted in a quartz fixed-bed microreactor with adiameter of 4 mm, containing 500 mg of catalysts in granules withparticle sizes from 20 to 30 mesh.

Methane and air were fed to the reactor so that the concentration ofmethane in the test was equal to 2% by volume. The tests were conductedat atmospheric pressure and at a spatial velocity, expressed as GHSV(gas hourly space velocity: hourly volumetric flow-rate in feed/volumeor weight of catalyst), equal to 80,000 cm³/gh and 55,000 cm³/gh,respectively. The mixture of the reaction products was analyzed by gaschromatography. The only products formed during the tests were carbondioxide and water.

The results of the tests at various spatial velocities were found to beidentical within the experimental error: the result obtained at 80,000cm³/gh is plotted in FIG. 1, where the term “conversion” references thepercentage in moles of converted methane with respect to the moles ofsupplied methane. The expression “set temperature” designates thetemperature of the mixture of gas fed to the reactor, and the term“measured temperature” designates the temperature of the gas mixture atthe outlet of the reactor, which is considered equal to the temperatureof the catalyst.

Methane conversion begins at a temperature from 300 to 350° C. Theresulting conversion curve has an inflection point at the temperature of500° C. Total conversion is achieved at 650° C. Throughout theconversion process, the only products obtained were CO₂ and H₂O.

From the point of view of molecule reactivity, methane is to beconsidered difficult to oxidize. The required reaction conditions aretherefore extremely drastic if compared with the other paraffins,olefins or volatile organic compounds. The tests that have beenconducted have shown that the compound according to the invention iscapable of oxidizing methane at relatively low temperatures: this is anindicator of the high full oxidation capacity of the inventioncompounds. As regards the other volatile organic compounds, one shouldassume that total combustion occurs at markedly lower temperatures thanthose found for methane.

Example 2

Ca₄Cu₅O₁₀: Catalytic Combustion of Methane

A catalyst containing Ca₄Cu₅O₁₀ was used in catalytic methane combustiontests. The tests were conducted in a quartz fixed-bed microreactor witha diameter of 4 mm, containing 500 mg of catalysts in granules withparticle sizes from 20 to 30 mesh.

The catalyst was a composite material constituted by an inert poroussubstrate (Al₂O₃) containing 7.5% by weight of active compound (38 mg).

Methane and air were fed to the reactor so that the concentration ofmethane in the test was equal to 2% by volume. The tests were conductedat atmospheric pressure and at a spatial velocity, expressed as GHSV(gas hourly space velocity: hourly volumetric flow-rate in feed/volumeor weight of catalyst), equal to 80,000 cm³/gh and 55,000 cm³/gh,respectively. The mixture of the reaction products was analyzed by gaschromatography. The only products formed during the tests were carbondioxide and water.

The results of the tests at various spatial velocities were found to beidentical within the experimental error: the result obtained at 80,000cm³/gh is plotted in FIG. 2, where the term “conversion” designates thepercentage in moles of converted methane with respect to the moles ofsupplied methane. The expression “set temperature” designates thetemperature of the mixture of gas fed to the reactor, and the term“measured temperature” designates the temperature of the gas mixture atthe outlet of the reactor, which is considered equal to the temperatureof the catalyst.

Methane conversion begins at a temperature from 350 to 400° C. Theresulting conversion curve has an inflection point at the temperature of550° C. Total conversion is achieved at 700° C. Throughout theconversion process, the only products obtained were CO₂ and H₂O.

Example 3

Sr₁₄Cu₂₄O₄₁: Tests of Methane Combustion in Oxygen Deficit Conditions

A catalyst Sr₁₄Cu₂₄O₄₁, fully similar to the one used in the test citedin example 1, was used in tests of methane combustion in oxygen deficitconditions. The tests were conducted in a quartz fixed-bed microreactorwith a diameter of 4 mm, which contained 200 mg of catalyst in granuleswhose dimensions were from 20 to 30 mesh. Methane, oxygen and helium ina proportion of 2:1:20 vol/vol were fed to the reactor. The test wasconducted at atmospheric pressure and at a spatial velocity, expressedas GHSV, equal to 80,000 cm³/gh. The mixture of the reaction productswas analyzed by gas chromatography. The products present in the gases inoutput at temperatures below 900° C. were carbon dioxide and water,generated in a stoichiometric quantity with respect to the fed oxygenand excess unreacted methane.

The results of the activity test are plotted in FIG. 3, where the term“conversion” designates the percentage of moles of converted methanewith respect to the moles of supplied methane. The amounts of methaneand oxygen used in the experiment allow a maximum conversion of 25% ofthe methane. The maximum value achieved and plotted in FIG. 3 is withinthe expected experimental error.

Example 4

Ca₄Cu₅O₁₀: Tests of Methane Combustion in Oxygen Deficit

A catalyst Ca₄Cu₅O₁₀, fully similar to the one used in the test cited inexample 2, was used in tests of methane combustion in oxygen deficitconditions. The tests were conducted in a quartz fixed-bed microreactorwith a diameter of 4 mm, which contained 500 mg of catalyst in granuleswhose dimensions were from 20 to 30 mesh. Methane, oxygen and helium ina proportion of 2:1:4 vol/vol/vol were fed to the reactor.

The test was conducted at atmospheric pressure and at a spatialvelocity, expressed as GHSV, equal to 80,000 cm³/gh. The mixture of thereaction products was analyzed by gas chromatography. The productspresent in the gases in output at temperatures below 720° C. were carbondioxide and water, generated in a stoichiometric quantity with respectto the fed oxygen and excess unreacted methane.

The results of the activity test are plotted in FIG. 4, where the term“conversion” designates the percentage of moles of converted methanewith respect to the moles of supplied methane. The amounts of methaneand oxygen used in the experiment allow a maximum conversion of 25% ofthe methane. The maximum value achieved and plotted in FIG. 4 is withinthe expected experimental error.

Example 5

Sr₁₄CU₂₄O₄₁: Conversion of CO to CO₂

The catalyst Sr₁₄Cu₂₄O₄₁ prepared as described in example 1 was used intests of conversion of carbon monoxide, CO, to carbon dioxide, CO₂. Thetests were conducted in a quartz fixed-bed microreactor with a diameterof 4 mm, containing 0.5 g of catalyst in granules having dimensions from20 to 30 mesh. Carbon monoxide, oxygen and nitrogen in a volumetricconcentration of 1%, 5% and 94%, respectively, were fed to the reactor.

The test was conducted at atmospheric pressure and at a spatial velocityof 50,000 cm³/hg. The mixture of reaction products was analyzed by gaschromatography, using helium as the carrier gas. The composition of theresulting mixture of gases was analyzed by measuring the concentrationsof CO and CO₂, which were found to be consistent with the reaction2CO+O₂→2CO₂.

The results of the test are plotted in FIG. 5, where the term“conversion” designates the percentage of moles of converted CO withrespect to the moles of supplied CO.

Example 6

Ca₄Cu₅O₁₀: Conversion of CO to CO₂

A catalyst containing Ca₄Cu₅O₁₀ was used in tests of conversion ofcarbon monoxide, CO, to carbon dioxide, CO₂. The tests were conducted ina quartz fixed-bed microreactor with a diameter of 4 mm, containing 0.5g of catalyst in granules having dimensions from 20 to 30 mesh. Carbonmonoxide, oxygen and nitrogen in a volumetric concentration of 1%, 5%and 94%, respectively, were fed to the reactor.

The catalyst was a composite material constituted by an inert poroussubstrate (Al₂O₃) containing 11% by weight of active compound (55 mg).

The test was conducted at atmospheric pressure and at a spatial velocityof 50,000 cm³/hg. The mixture of reaction products was analyzed by gaschromatography, using helium as the carrier gas. The composition of theresulting mixture of gases was analyzed by measuring the concentrationsof CO and CO₂, which were found to be consistent with the reaction2CO+O₂→2CO₂.

The results of the test are plotted in FIG. 6, where the term“conversion” designates the percentage of moles of converted CO withrespect to the moles of supplied CO.

The catalysts according to the present invention and the method for fulloxidation of VOC according to the present invention can be used withgood results for organic compounds which are gaseous or vaporize at lowtemperature, achieving complete combustion at low temperature. The CO₂selectivity of the catalysts according to the present invention is animportant characteristic. Oxidation of VOC, particularly hydrocarbons,with catalysts which are not specific for combustion usually leads tothe generation of both CO and CO₂. Both products are thermodynamicallyfavored in the conditions that are normally used, and the ratio istherefore usually conditioned by kinetic factors.

Accordingly, the specificity of the catalyst in the forming of CO₂ islinked to the characteristics of the active centers. CO₂ is in fact theprimary product, as demonstrated by the tests conducted by varying thecontact time, which show that the conversion does not depend on thespatial velocity, and therefore does not derive from the intermediateforming of CO. It is also important to note that the primary carbonmonoxide, i.e., the carbon monoxide that does not derive from partialoxidation of VOC, is converted efficiently into carbon dioxide attemperatures above 100° C. in the case of A₁₄Cu₂₄O₄₁ and 150° C. in thecase of Ca₄Cu₅O₁₀ in the presence of oxygen at a concentration which issufficient to allow the reaction.

The catalysts according to the present invention and the method for fulloxidation of gaseous organic compounds can be used to reduce oreliminate many noxious components of combustion gases generated in anymanner, for example from very large-scale electric power stations downto small combustors for home use, including the important applicationsin the field of engines and of vehicles with internal-combustionengines.

The selectivity of the CO₂ yield furthermore makes it particularlyinteresting to use the catalysts according to the present invention inburners for closed environments (gas stoves, water heaters for sanitaryuse, cooking equipment, et cetera), in which the presence of CO and incombustion products is notoriously a severe health hazard.

Furthermore, the catalysts according to the present invention,particularly the compounds of the Sr₁₄Cu₂₄O₄₁ type, convert the carbonmonoxide that is present in gas mixtures containing oxygen into carbondioxide in a very efficient way and at low temperature.

The catalysts for catalytic oxidation according to the present inventionare characterized by high activity, comparable to that exhibited by themore expensive noble metals and by the material Ba₂Cu₃O₆, by highresistance to temperature and to extreme operating conditions, and bylow cost and simple production even in the form of compounds and thinfilms.

Furthermore, like the material Ba₂Cu₃O₆, they have a selectivity forcarbon dioxide, CO₂, with respect to carbon monoxide, CO. The advantagesof the materials A₁₄Cu₂₄O₄₁ and B₄Cu₅O₁₀ with respect to the materialBa₂Cu₃O₆ relate to:

-   -   reduced limitation in the deposition of large quantities of        active material of the supported catalyst containing Ca or Sr        with respect to the catalyst containing Ba;    -   a very advantageous cost with respect to Ba compounds;    -   lack of toxicity of Ca and Sr with respect to heavy metals such        as Ba;    -   exclusively for the compound Sr₁₄Cu₂₄O₄₁, higher catalytic        activity in all full oxidation reactions for equal temperature        conditions.

The catalysts according to the present invention lead to full oxidationof VOC in stoichiometric mixtures of methane and oxygen or in excess ofoxygen (oxidizing conditions). In this manner, the mixture of gasesproduced by VOC oxidation contains no carbon monoxide but contains onlycarbon dioxide. In this manner, the operations for removing carbonmonoxide, and the known negative consequences of its presence in theenvironment, are avoided. Moreover, by oxidizing the carbon monoxide tocarbon dioxide, the entire potential of the VOC oxidation reaction isutilized, providing evident energy-related advantages.

The disclosures in Italian Patent Application No. BO99A000314 from whichthis application claims priority are incorporated herein by reference.

1. A catalyst for the full oxidation of volatile organic compounds (VOC)and of CO to CO₂, comprising: 5% to 20% by weight of anon-stoichiometric crystalline compound conventionally designated by aformula which corresponds to A₁₄CU₂₄O₄₁ (I), where A is Sr or a solidsolution of Sr with alkaline-earth metals, alkaline metals orlanthanides; or a non-stoichiometric crystalline compound conventionallydesignated by a formula which corresponds to B₄Cu₅O₁₀ (II), where B isCa or a solid solution of Ca with alkaline-earth metals, alkaline metalsor lanthanides; or mixtures thereof; that is prepared in a form whichhas a large specific surface area, preferably larger than 25 m²/g.
 2. Amethod for preparing a catalyst comprising a nonstoichiometriccrystalline compound conventionally designated by a formula whichcorresponds to Sr₁₄Cu₂₄O₄₁ comprising the steps of: a) immersing apre-dried granular porous substrate material in an aqueous solution witha molar concentration of Sr(NO₃)₂ from 0.23 M to 0.93 M and a molarconcentration of Cu(NO₃)₂ from 0.39 M to 1.59 M; b) drying the productof step a) at a temperature from 80° C. to 120° C.; and c) holding theproduct of step b) at a temperature from 650° C. to 750° C. in a gasstream which contains oxygen until complete decomposition of thenitrates occurs.
 3. The method according to claim 2, wherein the porousmaterial is selected from the group consisting of Al₂O₃, ZrO₂, CeO₂,TiO₂, and MgO.
 4. A method for preparing a catalyst comprising anonstoichiometric crystalline compound conventionally designated by aformula which corresponds to Ca₄Cu₅O₁₀ comprising the steps of: a)immersing a pre-dried granular porous substrate material in an aqueoussolution of Ca(NO₃)₂ and Cu(NO₃)₂ in an equimolar ratio and at a molarconcentration from 0.39 M to 1.39 M; drying the product of step a) at atemperature from 80° C. to 120° C.; and b) holding the product of stepb) at a temperature from 650° C. to 750° C. in a gas stream whichcontains oxygen until complete decomposition of the nitrates occurs. 5.A method for preparing a catalyst comprising a non-stoichiometriccrystalline compound conventionally designated by a formula whichcorresponds to Ca₄Cu₅O₁₀, comprising the steps of: a) immersing apre-dried granular porous substrate material in an aqueous solutionobtained by dissolving, with the application of heat, CuO and CaCO₃ innitric acid, so that the molar ratio between the components of thesolution is CuO:CaCO₃:HNO₃=1:0.83:3.2; water and citric acid being addedthereto so that the citric acid: Cu molar ratio is from 3.5:1 to 4.0:1;b) heating the product of step a) in air until combustion of the organicfraction of the absorbed material is achieved; and c) thermal treatingthe product of step b) for 4 to 24 hours at a temperature from 650 to750° C. in a stream of gas containing oxygen.